DESCRIPTION (Investigator's abstract): This application is intended to test the hypothesis that the fever, fatigue and myalgia experienced by some patients with chronic fatigue syndrome (CFS) are due to abnormal cytokine production and leukocyte activity. Cytokines such as interleukin 1B (IL-1B) and tumor necrosis factor-a (TNFa) mediate fever and metabolic adaptations during infection. Exercise enhances production of these cytokines in healthy individuals, but an altered response may account for the exertionally-induced clinical signs and symptoms in CFS patients. In this study, cytokine production, along with associated leukocyte functions and metabolic responses will be measured before and after exercise in CFS patients and control subjects. A demonstration of abnormal cytokine production after exercise could help to explain the pathogenesis of an illness that at present is poorly understood. In addition, such information may lead to effective therapy; cytokine production can be influenced by dietary modifications, and cytokine activity can be altered by receptor antagonists and soluble receptors that are currently undergoing clinical trials.